High-resolution small angle X-ray scattering (SAXS) measurements were performed on two series of poly(ethylene-alt-propylene)-b-poly(D,L-lactide) (PEP-PLA) diblock copolymer materials exhibiting differences in the widths of the poly(D,L-lactide) block molecular mass distributions as measured by their polydispersity indices (PDIPLA). At symmetric compositions of PEP-PLA (f(PLA) approximate to 0.5), all SAXS data were successfully fit to an established model describing the small angle scattering from lamellar mesostructures. According to this model, the increase in the PDIPLA negligibly affected the amount of lattice disorder. The apparent asymmetry of the poly(ethylene-alt-propylene)-block lamellae (phi), also determined by the fitting procedure, were more substantially affected; increasing the PDIPLA resulted in a decrease in phi. At asymmetric compositions of PEP-PLA (f(PLA) approximate to 0.67), only the data at the highest values of the PDIPLA could be reasonably fit to this model. (c) 2007 Wiley Periodicals, Inc.